The present invention relates to a binarizing method and apparatus therefor to be carried out before the position of a position correcting mark or that of a circuit pattern placed on a substrate is recognized by an image processing apparatus in semiconductor processing equipment.
In recent years, it has become necessary to align a substrate and a mask with each other in a projecting/exposure apparatus used in semiconductor processing equipment. To this end, a method of recognizing the position of a position correcting mark has been conventionally carried out by means of a pattern such as an image without making contact with the position correcting mark.
The above-described conventional binarizing method used to recognize the position of the position correcting mark or that of the circuit pattern is described below with reference to FIG. 3.
FIG. 3 is a descriptive view of the binarizing method in which the density of an image picked up by a television camera is expressed by frequency distribution.
If an object is a position correcting mark placed on a substrate and the density of the mark is higher than that of the substrate serving as the background surface, the frequency distribution has two mountains in which the mountain having the higher density indicates the density distribution of the mark and the mountain having the lower density indicates the density distribution of the substrate.
FIG. 4 is a construction view showing a conventional position recognizing apparatus used to carry out the conventional method.
An illuminating device 3 is installed at a position diagonally above an object 2, placed on a positioning table 1, having a predetermined pattern which is to be recognized. A television camera 5 is installed on a movable television camera supporting section 4 so that the television camera 5 picks up the image of the object 2. The television camera 5 is connected with a television camera control circuit 7 for controlling the operation of the television camera 5 in response to an instruction supplied from an operation panel and a controller through a decision control circuit (CPU) 6. The apparatus further comprises a window frame control circuit 8 for setting a range to be processed according to an object and an A/D conversion circuit 9 for converting an analog value of a video signal outputted from the television camera 5 into a digital value.
A binarizing apparatus 10 surrounded by a dotted line in FIG. 4 comprises a maximum/minimum density detecting circuit 11 for detecting the maximum density fmax and the minimum density fmin in the processing range of an image to be binarized; a division level ratio storing circuit 12 for storing a division level ratio .alpha. in dividing a density histogram in the processing range into two regions; a division level calculating circuit 13 for calculating a division level INiTBiN based on the division level ratio .alpha.; a representative density calculating circuit 14 for calculating the average density Ml and Mu of each of the two regions of the density histogram divided by the division level INiTBiN; a binarization level ratio storing circuit 15 for storing a binarization level ratio .beta. in determining a binarization level BiN; a binarization level calculating circuit 16 for calculating the binarization level BiN based on the binarization level ratio .beta.; a position recognizing circuit 18 for binarizing the image based on the determined binarization level BiN and then detecting the predetermined pattern of the image by means of template matching based on the binarized image and recognizing the position thereof; and a binarization level control circuit 17 for alterably setting the binarized level BiN by a predetermined density when the predetermined pattern is not recognized.
The binarizing method to be carried out by the apparatus of the above construction is described below with reference to a flowchart of FIG. 5.
First, at step 1, the maximum/minimum density detecting circuit 11 detects the maximum density fmax and the minimum density fmin in the processing range of the image picked up by the television camera 5. At step 2, the division level calculating circuit 13 takes a frequency distribution (density histogram) of the density and divides the density histogram into two regions based on the predetermined division ratio .alpha. stored in the division level ratio storing circuit 12. A threshold to be used in dividing the density histogram into the two regions is called a division level. The division level INiTBiN is found by the following equation 1: EQU Division value level INiTBiN=fmin+.alpha..times.(fmax-fmin) (1)
Of the two regions thus obtained, the region of the smaller density is denoted by A and the region of the greater density is denoted by B. At step 3, the representative density calculating circuit 14 calculates the average densities Ml and Mu which are the representative density of each region. ##EQU1## In the above equation 2, N(f) denotes a frequency when density level is (f).
At step 4, using the average densities Ml and Mu and the binarization level ratio .beta. previously set and stored in the binarization level ratio storing circuit 15, the binarization level calculating circuit 16 determines the binarization level BiN as follows: EQU Binarization level BiN=Ml+.beta.(Mu-Ml) (3)
At step 5, the position recognizing circuit 18 binarizes the image by using the binarization level BiN thus determined, thereby recognizing the position of the pattern.
If the position of the pattern is correctly recognized, the binarization level BiN is appropriate. If the position of the pattern is not correctly recognized, the binarization level BiN is inappropriate. At that time, at step 6, the binarization level control circuit 17 alters the binarization level BiN by the predetermined density and the position recognizing circuit 18 binarizes the image based on the altered binarization level BiN and thus recognizes the position of the pattern until the position of the pattern is correctly recognized. If the position of the pattern is not correctly recognized notwithstanding the repeated alteration of the binarization level BiN, it is decided that the binarization level BiN is not inappropriate, but that the state of the object is inappropriate.
However, according to the above-described conventional method, in an apparatus in which chemical reactions, for example, semiconductor processes take place, the states of circuit boards are chemically varied lot by lot and as a result, and the characteristic of the density histogram of an image such as a position recognizing pattern or mark nonlinearly changes. Therefore, it is necessary to alter the binarization level many times and consequently much time is required for image processing.